In hydraulic drives, the driving power is known to be controlled by the pressure and the flow velocity of a working fluid conducted in a circuit from a reservoir, usually formed as an oil pan, via the pressure side of a motor-driven pump to a hydraulic consumer and from the low-pressure side of the latter back to the reservoir. To conduct the working fluid in this cycle, hydraulic power conduits are provided in flow connection between each of said driving components. The pump is typically a hydraulic pump with constant or variable delivery volume, i.e. a screw pump or a radial piston pump, which is driven by an electric motor. Consumers to be used are hydraulic motors for generating a translatory or rotatory output movement; the former may be configured as hydraulic cylinders, the latter as gear motors. The working fluids used are usually fluids on the basis of mineral oil, so-called hydraulic oils, synthetic fluids, or fluids on the basis of plants, wherein the latter particularly stand out due to its environmental compatibility. These working fluids can contain additives that permit a selective influencing of individual characteristics, such as the thermal characteristic, the aging resistance, or the corrosiveness.
Hydraulic drives are characterized by its high power density, its high efficiency, and its simple continuous controllability of the output motion with a high positioning accuracy, and are used in both vehicles and stationary equipment. The hydraulic power conduits for conducting the working fluid both between the pressure side of the hydraulic pump and the pressure side of the hydraulic motor and also between the return side of the latter and the reservoir are made in a flexible and/or rigid design so that the volume flow of the working fluid is also transferable over greater distances without significant additional mechanical effort and with the possibility of a separate arrangement of the drive side and the driven side of the hydraulic drive, or rather, of the motor-driven pump and the consumer. Thereby, hydraulic drives can be largely adapted without any difficulty to almost all space requirements at low space consumption. This proves particularly advantageous when such a drive is used in a hydraulic drive system for an elevator insofar as the prime mover does not necessarily need to be arranged in the elevator shaft together with the hydraulic consumer, or hydraulic jack, but, where applicable, can be arranged remote from there too. In this respect, the hydraulic drive also can be described as a hydrostatic transmission comprising a hydraulic pump, a hydraulic motor, and a reservoir with the working fluid, and the flow connections provided between these elements for conducting the working fluid in a closed or open circuit. As flow connections, or rather, tubing connections for conducting the working fluid within such a hydraulic system, all flow connections known to be suitable as flow connections in such a hydraulic system may in principle be considered, i.e. both rigid tubing elements, such as tubes or recesses in a housing of a particular element of the system, and flexible tubing elements, such as hoses.
A hydraulic drive system comprises, in addition to the hydraulic drive, any other devices for a failure-free operation, such as devices for filtering and/or draining of the working fluid, and safety devices, such as pressure relief valves, a control device for the continuous or discontinuous control of the volume flow of the working fluid from the hydraulic pump to the hydraulic motor and from the latter back to the reservoir. The control device is connected by hydraulic control conduits to the hydraulic power conduits in such a way that the working fluid is controllable by the control device with regard to direction and volume to and from the hydraulic consumer, or rather, hydraulic motor without conveying direction reversal of the pump, in fact in a way that the volume flow of the working fluid toward the hydraulic consumer allows a continuous output movement in a first direction and from the hydraulic consumer off a corresponding output movement in a second direction. As the hydraulic power conduits, the hydraulic control conduits are made in a flexible and/or rigid design in a customary manner and differ in this respect from the former essentially by their smaller cross section only.
In the case of approximately constant output loads, the compressibility of the working fluid in hydraulic drive systems is negligible. With strongly varying loads acting on the hydraulic consumer, however, as is the case in a hydraulic drive system for an elevator, fluctuations with regard to pressure and output movement may occur owing to the elasticity of volume of the working fluid, whereby the desired consistency of the output movement, or rather, output speed and force no longer is reliable. By strong operational heating of the working fluid, changes in viscosity in the working fluid may occur in addition, having similar adverse effects on the operating characteristics of the hydraulic drive system. In particular, the desired continuity of the output movement of the hydraulic consumer may also be adversely affected by pump-induced fluctuations of the volume flow of the working fluid.
A control device for controlling the speed of a hydraulic motor of the type having features of the invention describes CH-A5-629 877. Thereafter, the speed of the motor is detectable via the flow of oil in the motor inlet by a suitable flow meter and convertible into an electrical signal, which can be supplied to a comparator for forming a control signal as a function of a target value signal to be preset for a control valve arranged in each the inlet and the outlet of the motor. For actuating the valve, an electro-magnetic transducer is provided in each case in the form of an electrovalve in electrical connection with the comparator. In this respect, the components mentioned represent a control circuit which is capable of effectively compensating for deviations from the target value. The known control device thus allows presetting of selected motor speeds and an effective correction of parasitic drags, such as fluctuations of pump pressure and load.
Indeed it is possible with such a control to overcome the aforementioned drawbacks of a hydraulic drive system. Nonetheless, the latter has also disadvantages. Thus, it notably is disadvantageous that for the actuation of the two control valves in each case a separate electric actuator in the form of an electro-magnetic transducer in each case coupled to a particular pilot valve in each case is required, in order to ensure the desired direction-dependent control of the volume flow of the working fluid in the connection line on the high-pressure and low-pressure side of the hydraulic consumer, and thereby the correction of parasitic drags. The use of two electric actuators, however, is not only reflected in the manufacturing and the operating costs of such a control device, but also accounts for an increased risk of default, which in turn reduces the reliability of the hydraulic drive system as a whole.